Ultraviolet rays are essential for the biosynthesis of vitamin D and also used to promote blood circulation or metabolism in the living body and to achieve sterilization or disinfection. On the other hand, excessive exposure of the skin to ultraviolet rays can cause skin cancer or promote skin aging which can cause freckles and wrinkles. Excessive exposure to ultraviolet rays can also cause the degradation of paints and a variety of synthetic resins such as polyethylene, polypropylene, PVC, and ABS resin. Thus, excessive exposure to ultraviolet rays can result in adverse effects.
To prevent such adverse effects, a variety of ultraviolet-absorbing agents have been developed and widely used.
Near ultraviolet rays usually called “ultraviolet rays (UV)” are broadly classified into UVA (315 to 400 nm in wavelength), UVB (280 to 315 nm in wavelength), and UVC (200 to 280 nm in wavelength). UVC, although derived from sunlight, is substantially absorbed by the ozone layer, etc., before reaching the earth, and thus usually does not reach the earth through the ozone layer, etc.
UVA and UVB are the main cause of skin cancer, skin aging, and the degradation of synthetic resins and paints.
Thus, ultraviolet-absorbing agents are developed for the purpose of absorbing mainly UVA or UVB.
Known examples of ultraviolet-absorbing materials contained in conventionally developed ultraviolet-absorbing agents include isooctyl p-methoxycinnamate, isoamyl p-methoxycinnamate, sodium phenylbenzimidazole sulfonate, 3-(4′-methylbenzylidene)-camphor, 4-tert-butyl-methoxy-dibenzoylmethane, and 4-isopropyl-dibenzoylmethane. These compounds all have an aromatic ring. The conjugated electrons in the aromatic ring can be excited with energy lower than that for non-conjugated electrons and thus can absorb ultraviolet rays. In addition, when the conjugated system is expanded, the electrons in the conjugated system can be excited with lower energy and thus can absorb long-wavelength ultraviolet rays and also visible rays.
Many UVB absorbing compounds suitable for practical use have been developed. Known examples of such UVB absorbing compounds include isooctyl p-methoxycinnamate, isoamyl p-methoxycinnamate, sodium phenylbenzimidazole sulfonate, and 3-(4′-methylbenzylidene)-camphor mentioned above.
On the other hand, many of conventional UVA absorbing compounds are decomposed in the process of converting the light energy of absorbed UVA into heat energy and releasing the heat energy. This means that such conventional UVA absorbing compounds have a problem with light stability, and at present, still few compounds have been developed to solve this problem.
Examples of currently developed UVA absorbing compounds include the hydantoin derivatives disclosed in Patent Documents 1 and 2.
When compositions for external use to skin, such as sunscreen compositions, are designed for use in water-contact activities such as sea bathing, they are required to have such properties that they can remain on skin parts where they are applied even when the skin parts comes into contact with water. Therefore, UVA absorbing compounds to be contained in such compositions should preferably have high oil solubility.
On the other hand, recently, as ultraviolet-induced skin damage and other complications have become well known, compositions for external use to skin such as sunscreen compositions have come to be used also in daily life (in other words, in situations where skin parts to which they are applied do not necessarily come into contact with water). For such applications, UVA absorbing compounds contained in compositions are required to have high hydrophilicity so that the compositions can be free of sticky feeling, crustiness, or white residue during use and also can be easily washed off from skin parts after use (good removability).
The hydantoin derivatives disclosed in Patent Documents 1 and 2 are too low in hydrophilicity to be used in such compositions for external use to skin. Compositions for external use to skin containing such derivatives can hardly fulfill these functions at the same time.
Thus, conventional azolidine derivatives have room for improvement because they cannot exhibit good hydrophilicity simultaneously with high ability to absorb ultraviolet rays (particularly UVA).